The invention relates to telecommunication devices, and in particular to apparatus and method of encoding and decoding Telephone Device for the Deaf (TDD) signals.
Telephone Devices for the Deaf (TDD transceivers) are used in communicating between two Public Switched Telephone Network (PSTN) terminations when voice can either not be understood or not possibly be transmitted such as is the case as one of the parties has at least a hearing or speech impediment respectively.
TDD transceivers are typically stand alone devices having a keyboard, a display and employ TDD signaling. TDD signaling is a term largely referring to the Baudot code which uses a 5 bit character encoding technique. A typical TDD terminal operates at 45.45 baud employing a carrier-less Frequency Keying as a modulation technique. The TDD transport protocol is asynchronous and uses 1 start bit, 5 data bits, and 1.5 stop bits. Each conveyed symbol represents a single bit. The bit representation includes a 1400 Hz tone (1) or a 1800 Hz tone (0). Silence between characters is tolerated. The carrier-less characteristic of the TDD signals enables TDD calls to be put on hold and/or transferred.
The field of communications is replete with transitive designs and data transfer protocols. In spite of that fact, TDD transitive technology has remained largely unchanged since its introduction.
Only a relatively small number of TDD transceivers exist in use. This is due in part to the fact that only a small fraction of the population is affected by hearing and/or speech impediments. Due to the relatively small market for TDD transceivers, the prices of these devices are very high compared to a telephone set. The high price of TDD devices has affected the market penetration of these devices. Market penetration of TDD devices is significantly less percentage wise when compared to the penetration of telephone sets.
The greatest disadvantage resulting from the lack of market penetration or wide spread use of TDD is the inability of friends and associates of people with hearing or speech impediments from communicating with the latter. Although recent trends have brought about legislation imposing equal access to all services by all persons regardless of disabilities, most small companies, and still some larger companies, do not provide customer access to services via TDD connections. Although the market is small on both sides, users and service providers would benefit from an increase in market penetration of these devices.
The obstacles to acquiring TDD capability are both a matter of cost for the specialized equipment as well as integration of the specialized equipment within existing telecommunications/network equipment.
Novel methods of providing TDD services have been previously proposed. U.S. Pat. No. 5,905,476 entitled xe2x80x9cITU/TDD MODEMxe2x80x9d which issued to McLaughlin et al. on May 18th, 1999, proposes the reuse of a modified modem. Although this proposition has merit, the solution proposed essentially is a modified modem which introduces a few problems.
Modems and modem protocols typically use modulation techniques employing carrier frequencies in transporting data and maintaining connections. This attribute makes it impossible for a modem connection to be put on hold and/or transferred because losing the carrier frequency is commonly attributed with a loss of connectivity and the modem disconnects. McLaughlin et al. provides solutions for this problem at the cost of an increased complexity by the use of special firmware and/or the use of an extended data transfer protocol leading to increased manufacturing and perhaps maintenance costs.
Another problem stems from the fact that companies are very sensitive to the addition of modems to their computers as a result of a perceived security threat. TDD communications are an essential alternative path for voice communications, which do not pose any security risk. Thus, the integration of modem communications to handle TDD communications presents a need to manage and control this possible security risk created by the connection of a modem to a company""s network. McLaughlin et al. provides solutions for this problem at the cost of an increased complexity special firmware and/or the use of an extended data transfer protocol leading to increased manufacturing and perhaps maintenance costs.
Therefore there is a need to provide novel apparatus and methods for supporting TDD telecommunications at a reasonable cost while adhering to common protocols, standards and corporate security needs.
It is an object of the invention to reuse a sound card, typically provided with a computer as purchased or available at a minimal cost, as a TDD transitive.
It is another object of the invention to enable TDD telecommunications via a sound card connected to a local loop using a simple adapter typically only having dynamic range matching components necessary in coupling a local loop to the soundcard.
It is yet another object of the invention to enable TDD telecommunications via a sound card such that the only modification required to a computer having an installed sound card coupled to a telephone line is a software installation upgrade.
According to a broad aspect of the invention a Telephone Device for the Deaf (TDD transitive) adapted for TDD signaling is provided. The TDD transitive comprises: a sound processor adapted to sample a received audio signal and generate a transmit audio signal, each audio signal having a frequency; and at least one correlator adapted to discriminate between at least two states representing samples derived from the received audio signal.
According to another aspect of the invention a TDD transitive further comprises a POTS adapter enabling the sound processor to operatively interface with a local loop.
According to a further aspect of the invention the POTS adapter further comprises an audio signal conditioning component adapted to match local loop characteristics to audio input and audio output characteristics of the TDD transitive; and a POTS connector operatively connected to the audio signal conditioning component adapted to receive the local loop.
According to yet another aspect of the invention the POTS adapter further comprises an audio coupler adapted to interface with and convey audio signals to and from a telephone handset.
According to another broad aspect of the invention a method of decoding a TDD coded frequency keying modulated received audio signal is provided. The method comprises a sequence of steps. The received audio signal is sampled and at least two frequencies discriminated.
According to another aspect of the invention, the method further comprises the step of assigning one of a first and a second logic states corresponding to one of the two frequencies based on a level of correlation between a group of samples representing the received audio signal and one of the first and second reference audio signals in extracting data bits from the received audio signal.
According to yet another aspect of the invention, the method further comprises extracting a character from the received audio signal. A start bit is detected upon assigning the first logic state to a group of samples following silence. preset number of data bits are detected and assigned logic states corresponding respective groups of samples representing the received audio signal. And, a stop bit is detected upon assigning the second logic state to a group of samples following the detection of the preset number of data bits.